You need to make the cylinders you are subtracting a smidge taller than what you subtract them from and shift them down half that smidge.  Like so.  (You don't need the use statement for this part at least.)

diameter=25.6/2;
tykkelse=1.5;

baselen=25;

centerhul=10 - 0.1;

module studs(){
studs_d = 25.6/2;
studs_l=30;

difference(){
    union(){
cylinder(h = studs_l, r1 = studs_d, r2 =studs_d);
translate([0,0,studs_l])
    cylinder(h = 4, r1 = studs_d+1, r2 =studs_d);
translate([0,0,studs_l+4])
    cylinder(h = 4, r1 = studs_d+1, r2 =studs_d);
translate([0,0,studs_l+8])
    cylinder(h = 4, r1 = studs_d+1, r2 =studs_d);
}
translate([0,0,-0.05])cylinder(h = studs_l+12.1, r1 = studs_d-3, r2 =studs_d-3);

}
}

// Tube
translate([-40,0,0])
rotate([0,90,0])
difference(){
cylinder(h = 120, r1 = diameter, r2 =diameter);
translate([0,0,-0.05])cylinder(h = 120.1, r1 = diameter-3, r2 =diameter-3);
}

studs();
translate([40,0,0])
studs();
translate([80,0,0])
rotate([0,90,0])
studs();
translate([-40,0,0])
rotate([0,-90,0])
studs();

Lenore

It appears you are trying to create an assembly from pipes using a module
to create the segments.  There's no way to make that work in base OpenSCAD,
but it is possible using BOSL2, as shown in example 6 for diff():

https://github.com/revarbat/BOSL2/wiki/attachments.scad#module-diff

On Fri, May 5, 2023 at 5:54 AM Kjeld Flarup kjeld.flarup@liberalismen.dk
wrote:

… and you have to cut the shapes of the vertical studs in the
horizontal tube:

use <MCAD/regular_shapes.scad>

diameter=25.6/2;
tykkelse=1.5;

baselen=25;

centerhul=10 - 0.1;

module studs(){
studs_d = 25.6/2;
studs_l=30;

difference(){
    union(){
cylinder(h = studs_l, r1 = studs_d, r2 =studs_d);
translate([0,0,studs_l])
    cylinder(h = 4, r1 = studs_d+1, r2 =studs_d);
translate([0,0,studs_l+4])
    cylinder(h = 4, r1 = studs_d+1, r2 =studs_d);
translate([0,0,studs_l+8])
    cylinder(h = 4, r1 = studs_d+1, r2 =studs_d);
}

// cylinder for cutting should overlap on both sides
translate([0,0,-1])
cylinder(h = studs_l+12+2, r1 = studs_d-3, r2 =studs_d-3);
}
}

// Tube
difference(){
union(){
// cutting the studs shape too
studs();
translate([40,0,0])
studs();
translate([-40,0,0])
rotate([0,90,0])
cylinder(h = 120, r1 = diameter, r2 =diameter);
}
// cylinder for cutting should overlap on both sides
translate([-92,0,0])
rotate([0,90,0])
cylinder(h = 222, r1 = diameter-3, r2 =diameter-3);
}

translate([80,0,0])
rotate([0,90,0])
studs();
translate([-40,0,0])
rotate([0,-90,0])
studs();

Peter

On Fri, 5 May 2023 06:05:17 -0400
Lenore Horner lenorehorner@sbcglobal.net wrote:

A simple method for tubular shapes, is to create the core with
cylinders/whatever, then difference it from a minkowski () copy of the
core and chop off the ends.

something like the following

module inner(){  // shape of core
   translate([30,0,0])cylinder(h=30,d=10);
   translate([60,0,0])cylinder(h=30,d=10);
   rotate([0,90,0]) cylinder(h=100,d=10);
       translate([30,0,0])bevel(10,10,3);
       translate([60,0,0])bevel(10,10,3);
}

//inner();

module all(){
    intersection(){  // trim to size
       translate([50,0,-20])
        cube(80,true);

      difference(){
        minkowski(convexity=10){
           inner();
           sphere(d=3); // make wall ( 1.5)
        }
          inner();
     }
   }
}

all();

module bevel(ld,td,th){  // gives a bit iof strength to corners

//ld = long diameter
//td = T off diameter
//th = T height from edge of long piece
  hull(){
    cylinder(h=th+(ld/2),d=td); // length of T to centre
     translate([-td/2-th,0,0])
     rotate([0,90,0])
     cylinder (h=td+th+th,d=ld);
  }
}

On 05/05/2023 10:53, Kjeld Flarup wrote: